The purpose of this project is to measure, via force-instrumented endoprostheses, the force vector at the human hip during surgery, recovery, rehabilitation, and in the activities of daily living (ADL). During rehabilitation and ADL concurrently, acquire precision body-segment kinematic foot-floor force and EMG data will be acquired. Patient-specific segment mass and inertial properties from computer-tomographic data will be generated and used in an inverse Newtonian calculation to estimate the force vector at the hip. The direct measurement will be compared with the gait-analysis-derived estimates which reflect only the NET muscle torques at the hip. Thus the applicants will quantify the contribution of agonist-antagonist muscle co-contraction during different movements. Using a musculoskeletal model of the lower-extremity and the gross force vector at the hip, they will re-evaluate the force versus time activity of individual muscles about the hip and compare with EMG data. Several hundred thousand major procedures are performed on the human hip annually in the U.S. alone, including repair and replacement of the femoral head, total joint replacement, and osteoarthritis. Attempts to measure the force vector at the hip have yielded limited data for only a brief period. Estimates of the force (net) from gait analyses vary widely (1 to 7 Body-weight during the stance phase of level walking). A pressure-instrumented endoprosthesis design, which has produced in vivo data for now over four years, will be augmented with a force-measuring element. Six units will be implanted over three years with data acquired in OR, recovery, and rehabilitation, with ADL follow-up for five years. Prosthesis design, hip surgery, patient management, rehabilitation protocols, and ADL advice would benefit from precise knowledge of the hip joint force. The presence and extent of co-contraction in movement will influence medical decisions and contribute to the understanding of the brain's control of movement.